The present invention relates in general to systems for displaying the operating parameters of power plants and, in particular, a new and useful method and apparatus for aiding the operation of a nuclear power plant during abnormal transient conditions.
Operator aids, in the form of displays, have been in use in power plants for a very long time. Indeed, the earliest power plants incorporated operator aids through use of improvised marks on water glasses, rag flags placed on vent lines, etc. Over the years, the art of providing operator aids has undergone many iterations. Today, this art is taking on new dimensions as computers are factored in.
With the coming of computers, a myriad of displays, commonly known as operator aids, have been made available. For the most part, these aids are passive and serve only to supply additional information to the plant operator. Currently, nuclear power plants utilize an operator aid known as the Safety Parameter Display System (SPDS). This SPDS is intended to relieve the operator of some diagnostic burden during periods of plant upset; this is generally a period of high stress.
When a power plant undergoes an abnormal transient, the plant operators are confronted with a complex decision making process. Recognition of this complex process led to the development of the first pressure-temperature (P-T) displays used as operator aids. This occurred during the conceptual design stage of vendor designed emergency operation guidelines (1979-1980). It was determined, at that time, that the integration of certain important data into one display could ease the operator's determination of what symptom was occurring and therefore what action to take. Operators found these displays an excellent diagnostic tool for use during plant upsets, as well as symptom identification during normal transients.
Later, it was decided that other information which would be helpful to operators during upset periods should be added to these displays. This led to the addition of protection system envelopes to these displays. Subsequent to this time, supplement 1 to NUREG-0737 was issued. This document referred to a Safety Parameter Display System (SPDS). In an effort to supply a match to this NUREG guidance, vendors added more information to the P-T displays and they were dubbed the SPDS. This is the current state of affairs relative to P-T display development.
Since the implementation of these displays, there have been several severe transients at nuclear power plants. In at least two of these transients, operators were not successful in determining when to take further, more extreme actions, they procrastinated beyond the point where action should have been taken.
In the mitigation process of any transient, there is necessarily a diagnosis and mitigation action time interval. That is, a diagnosis is made and then the appropriate action, based upon the diagnosis, is taken. Depending upon the complexity of the transient, there may be from one to very many of these diagnosis-mitigation time intervals (DM-TIs). Taken in the aggregate, these DM-TIs provide a measure of the time required to mitigate a given transient (includes time to reach stable plant conditions with normal controls in effect). Conceptually, the total transient time may be thought of as the sum of these DM-TIs, i.e. ##EQU1## The length of any DM-TI is subjective. It is based upon the operators ability to determine whether a particular action taken has been effective or not and when to continue on and take another more drastic appropriate action. Each DM-TI starts with the operators scan of vital instruments (including the SPDS) for the purpose of diagnosing plant conditions (given a symptom has been identified) and ends when an action has been completed or the transient has been mitigated, resulting in the operator's recognition of plant stability. Past experiences indicate that operators sometimes have difficulty determining if actions taken have been effective or not. Because of this, they may continue to diagnose plant conditions and proscrastinate before taking further more extreme actions. This necessarily increases the present DM-TI which in turn prolongs the overall transient time. Even more significant is the fact that procrastination in taking further action can lead to a transient of greater magnitude with its attendant consequences.
A major concern to regulators (NRC-INPO) of some vendor based Emergency Operating Procedures (EOPS) is the selection of appropriate procedures once a transient begins. The following paragraph from a recent INPO audit of NPP (nuclear power plant) EOPs sums up this point:
"The plant EOPs have little information in their diagnostic section to help the operator determine which transient is occurring. The information that is provided to the operator however, rarely references the P-T traces. In addition, most plant EOPs do not provide descriptions of the entry conditions at the start of each subprocedure, and they seem to rely heavily on the operator's ability to correctly identify a transient type (e.g., over-cooling) and select the proper subprocedure with a minimal amount of procedural direction." PA0 "This delayed their entering the proper procedure, and, in one case, an operator began executing an incorrect procedure before entering the correct one."
This report also points out that during observed simulator exercises operators had difficulty deciding which subprocedure they should execute.
Hench U.S. Pat. No. 4,421,716 et al discloses an apparatus for monitoring critical systems of a plant and provides information as to the status and crucial systems in summary form and upon request to a plant operator. The primary display of this reference has means for prompting the Operator as to which one of a number of push-buttons to press when an abnormal condition occurs. This message informs the Operator that further information can be obtained by pressing one of the push-buttons. The Operator is not given positive guidance on appropriate actions to take, however, nor a time frame for such actions.
Mlyaczak U.S. Pat. No. 4,657,727 et al discloses means for classifying emergency events. It identifies three fission product barriers which are intended to prevent the uncontrolled release of radioactive material to the environment. It establishes functional performance criteria for each fission product barrier.
Plant emergency symptoms are recognized by a computer or by the operators. The operators then implement emergency operating procedures to mitigate the symptoms of the emergency. Symptoms which are indicative of conditions which cannot be mitigated prior to the breach of a fission product barrier are identified and the computer automatically alerts outside authorities and notifies them of the emergency classification determined by the process.
Twilley, Jr. U.S. Pat. No. 4,608,223 discloses a system for monitoring a limited number of operating conditions in nuclear reactors. The system identifies abnormal operating conditions in a pressurized water reactor nuclear power plant and monitors a limited number of specific operating conditions which are common to post-reactor trip transient control. The system parameters remain within a "post-trip window" on a graphic display if the power plant is operating normally. When any of the foregoing system parameters falls outside this "window", this is an immediate indication to the power plant operator to institute the necessary procedures to remedy the same.
Book U.S. Pat. No. 4,568,513 et al teaches a system for monitoring and displaying operating parameters of a nuclear reactor which pertains to core power distribution. A cursor on a visual display indicates positions of the axial power shaping rods. Superimposed on a display is the operational limit line which is determined by the power PT margin systems.
Barry U.S. Pat. No. 4,292,129 teaches an apparatus for monitoring the processes of the nuclear reactor. Giras U.S. Pat. No. 4,258,424 et al relates to systems and methods for operating steam turbines and to electric power plants in which generators are operated by steam turbines. Musick U.S. Pat. No. 4,080,251 discloses a method and apparatus for calculating operating units for nuclear reactors which provides the margin which must be maintained in order to allow operation of the nuclear reactor in a safe manner.
Graham U.S. Pat. No. 4,079,236 et al discloses a method and apparatus which employ parameters that are monitored outside the reactor core.
Musick U.S. Pat. No. 4,016,034 is directed to a safety system for a nuclear reactor. The system overrides the nuclear reactor's controls with the purpose of insuring that minimum conditions are present at all times in order to ensure the adequate preparation of the nuclear reactor safety system. In essence, this prevents the operation of a nuclear reactor in a prohibited power configuration by limiting the power distributions which may be created in the core of the reactor.
Musick U.S. Pat. No. 3,998,693 teaches a monitoring system for providing warning and/or trip signals indicative of the approach of the operating conditions of a nuclear steam supply system. The thermal limit locus is calculated via signals representing the reactors cold leg temperature and core power. Then, the core power signal is adjusted to compensate for the effect of both radial and axial peaking factors.
Liang U.S. Pat. No. 3,873,817 discloses a system for monitoring and evaluating the turbine system, components and instrumentation performance in a nuclear fueled electric power generating system.
None of the foregoing references are able to provide to a plant operator a positive indication of what actions are appropriate in response to transient conditions and when the DM-TI has ended so that more extreme actions can be taken. These references thus do not avoid the extensive amount of subjective reasoning required on the part of a plant operator and possibly leading to periods of procrastination.